Human topoisomerase I (Topo I) is an enzyme critical to the viability of cellular function that is an attractive target for the design and development of anticancer therapeutics. Currently there are two anticancer agents approved by the Food and Drug Administration for the clinical treatment of cancers: topotecan (Hycamtin) and CPT-11 (Camptosar), both of which are structural analogues of the natural product camptothecin.
Eukaryotic DNA Topoisomerase I (Topo I) is an essential nuclear enzyme responsible for the organization and modulation of the topological dilemmas in DNA, such as overwinding, underwinding and catenation. Topo I plays a critical role in allowing a cell to appropriately replicate, transcribe, repair genetic information, and perhaps carry out other DNA processes such as chromatin assembly, recombination and chromosome segregation.1,2 
Topo I is a 100 kD monomeric protein that catalyzes changes in the topological state of double-stranded DNA (dsDNA) in increments of one linking number.3 The three-dimensional structure of human Topo I has been reported.4 The mechanism by which Topo I acts is believed to proceed through induction of a transient single-stranded break in dsDNA via formation of a covalent protein-DNA adduct referred to as the cleavable complex, so named because these complexes are detected as DNA breaks upon treatment with denaturing agents or alkali. The cleavable complex is formed upon transesterification of a DNA phosphodiester linkage by the active site tyrosine-723 residue on human Topo I, resulting in an ester linkage between the enzyme and the 3′-phosphoryl end of the broken DNA strand. This allows free rotation of the protein-bound 3′ end of the broken DNA strand about the intact complementary DNA strand, resulting in relaxation of the duplex in increments of one linking number. Religation of the broken strand (via a second transesterification reaction) and subsequent dissociation of topoisomerase I completes the catalytic cycle.
Topoisomerase I poisons act via stabilization of the cleavable complex, mediated by formation of a ternary complex consisting of drug, topoisomerase I and DNA.5 Agents such as camptothecin (the prototype topoisomerase I poison) do not bind to DNA directly, nor to topoisomerase I alone, but only to topoisomerase I complexed with DNA. It has been postulated that the stabilized DNA-protein-drug complex causes lethal DNA strand breaks upon collision with the advancing replication fork. It is by this mechanism that the topoisomerase I poison converts the enzyme into a DNA damaging agent, resulting in disruption of DNA replication and, eventually, cell death. This postulate is supported by the fact that camptothecin is highly phase-specific, only killing cells in S-phase.
It has been reported that intracellular levels of topo I are elevated in a number of human solid tumors, relative to the respective normal tissues, suggesting that variations in topo I levels are tumor type specific.6-8 Thus, topo I represents a promising target for the development of new cancer chemotherapeutic agents against a number of solid tumors. Development of anti-topo I agents offers a new approach to the multi-regimental arsenal of therapies currently used in the clinic for the treatment of cancer.
In addition to the camptothecins, indolocarbazoles have also demonstrated potent antitumor activity via the poisoning of topoisomerase I activity,9-12 most notably ED-110,13 NB-506,14 and J-107088.15 The indolocarbazole analogue bearing a 3,9-dihydroxy substitution pattern was found to have superior topoisomerase I poisoning capability as well as superior in vitro antitumor activity relative to the other “symmetrical” dihydroxylated regioisomers.16 The 3,9-dihydroxy analogue also exhibited impressive in vivo antitumor activity against the DU-145 human prostate tumor line xenotransplanted into nude mice.